This invention concerns a pneumatic device to measure the dimensions of a workpiece, and in particular to a device for taking measurements according to the differential pressure measurement procedure defined by standard DIN 2271.
As can be seen in FIG. 1, this procedure consists of feeding through a pressure regulator 1 a pneumatic circuit containing a first branch 2 equipped with an inlet nozzle 3 and a measuring nozzle or spindle 4, capable, for example, of being inserted in a bore 5, the diameter of which is to be measured, as well as a second branch 6 equipped with an inlet nozzle 7 passing out into a reference nozzle 8, the delivery of which can be adjustable. A pressure transducer 9 connected between branches 2 and 6 delivers an electric signal representing the difference in pressures prevailing in those branches, which is amplified by an amplifier 10 before being supplied, for example, to an indicator 11. It is thus clear that if the spindle 4 is inserted in a larger bore, the pressure in branch 2 will be reduced, and the variation of differential pressure detected by the transducer 9 will represent the increase in diameter of the bore 5. The essential value of the differential system is to absorb automatically the parasite pressure variations due, for example, to operation of the pressure regulator 1 or to the effects of temperature.
In the known devices,a transducer is used in which, for example, the displacement of a diaphragm is detected by an inductive pick-up connected to the latter. However, in the signal delivered by that type of transducer, a noise signal can exist which is superposed on the signal representing the dimension measured and the resulting large amplitude prevents known systems from performing precision measurements.
Furthermore, known systems are generally very sensitive to vibrations because of the presence of moving mechanical parts in the transducer, which can lead to disturbances or errors in measurement. That is why it is necessary to house most of the elements of the pneumatic circuit, including the inlet and reference nozzles as well as the transducer, in a separate box, to which the measuring spindle is connected by a duct. This houses a large volume of air which considerably lengthens the time constant of the measuring device.
Still another disadvantage of the known devices is that they are difficult to adjust. In particular, adjustment of the sensitivity of measurement is obtained by changing the geometry of the nozzles, which can lead in turn to a displacement of the linear characteristic of the measuring device and of the zero of the electric measuring signal.
That is why an object of this invention is to provide an improvement of the known devices, which considerably reduces the measurement noise, as well as the time constant and sensitivity to vibrations, while allowing a simple and independent adjustment of the sensitivity of the device and of the zero of the output signal.